1. Field of the Invention
This invention relates to an image processing apparatus, and more specifically to an image processing apparatus which can improve the quality of the output image.
2. Related Background Art
A known image processing apparatus, such as a copying machine, a facsimile apparatus, etc., is used for converting dot resolution in an image.
Such a known image processing apparatus is structured to print recording data (data to be recorded) in such a way as simply to increase the size of a dot by four times, in a case where a printer designed to operate at a dot recording resolution of 400 dpi (dots per inch) is used for printing recording data of 200 dpi, for example.
However, the known image processing apparatus suffers from the problem that the whole output image is darkened, or what should be a thin line of the output image is excessively wide because four times the size of dot is used for printing notwithstanding the printer has an ability of 400 dpi dot resolution.
The apparatus further has a defect that printing density is darkened in such a manner as to give the output image a strange appearance in the case where an isolated dot of, e.g., 200 dpi size, processed by an artificial halftone processing method, is printed.
Especially, a laser beam printer using an electrophotographic method of printing has the defect that an increase of a darkened rate is conspicuous because of a fogging phenomenon which occurs due to the overlapping and combination of adjacent dots, or because of the round shape of the beam spot.
In recent years, an error diffusion method has been proposed as an artificial halftone processing method. When a laser beam printer records an image based on binary data processed by the error diffusion method, there is a possibility that dots may be linked in a highlight portion because the shape of the dots is round and the size of the dots is large. This has resulted in the defect that what should be a thin line in the output image is made to seem fat, or sometimes, a peculiar striped pattern occurs in the output image.
Methods for obtaining a fine line of a proper width and generally for obtaining a proper reproduced image are disclosed in U.S. Pat. Nos. 4,387,983; 4,476,474; 4,517,579; 4,878,068; and 4,905,023, assigned to the assignee of the present invention, but further improvement of those methods has been sought.